AZquotes claims that Lord Kelvin (or William Thomson) said this:
If you can not measure it, you can not improve it.
But there is no such a quote in his Wikiquote page. So did he actually say this?
He didn't say that exact quote; Lord Kelvin tended towards rambling, so if you see any particularly pithy one-liner quotes, it probably wasn't him. His actual quote?
I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.
The actual source is from Antoine-Augustin Cournot, in De l’origine et des limites de la correspondance entre l’algèbre et la géométrie (1847), 375.